This invention relates to linear motors forcers. Well known in the art are linear motor systems of the type known as Sawyer motors after the original inventor Bruce Sawyer. These systems consist of a flat strip or sheet of magnetic material, such as electrical grade steel, with periodic grooves cut or etched in the surface forming teeth, and an opposed mechanism with toothed electromagnetic elements. The steel strip or sheet is referred to as a platen, while the electromagnetic mechanism is referred to as a forcer.
If the Sawyer motor operates in only one direction, it is a linear motor. The platen in this case is a strip with teeth cut in a direction orthogonal to the direction of motion. The forcer for a linear motor may contain only a single set of electromagnets, although multiple sets may be used to increase force. If the Sawyer motor operates in two directions it is a planar motor. The platen in this case is a sheet with teeth cut in two orthogonal directions forming a grid of intersecting grooves. The forcer for a planar motor must contain at least two electromagnets, one for each axis of motion, although more electromagnets are typically used. More specialized versions of both linear and planar Sawyer motors exist which use curved electromagnets and curved or cylindrical platens. Such designs can provide linear as well as rotational motion from a single motor.
The grooves formed in both the platen surface and the forcer electromagnets, are typically filled with a strong, stable epoxy, and both the platen surface and the toothed forcer surface are ground and lapped to provide an air-bearing quality surface. The forcer is provided with a flexible cable assembly, termed an umbilical, which contains the motor coil leads and a compressed air supply tube. Passages in the forcer mechanism convey the compressed air supplied by the tube to an arraignment of small orifices or air jets, releasing the air into a thin gap which forms between the forcer and the platen in opposition to the strong magnetic attraction between these two members. Planar Sawyer motors almost universally utilize air-bearings. Linear Sawyer motors may utilize air-bearings, ball or roller bearings, or recirculating ball slide units.
It is a characteristic of Sawyer motors that the electromagnetic mechanism contains all permanent magnets and all coil windings, while the platen is fundamentally a passive magnetic element. Numerous other linear and planer motor designs exist wherein the platen may contain permanent magnets or coil windings. These are usually referred to as brushless linear motors, moving coil motors, or AC linear motors.
This invention relates specifically to the design of hybrid variable reluctance electromagnets used in linear motor forcers and is not concerned with the details of linear motor platens. A linear motor or planar motor consists of the combination of both a forcer and a platen. The remainer of this disclosure will use the term forcer in referring to the invention when not specifically discussing a forcer and a platen.
Numerous patents and technical publications have disclosed the prior art in this field. U.S. Pat. No. 3,376,578 to Sawyer described various 3-phase variable reluctance magnetic structures for forcers. This patent also disclosed the general equation for spacing single-phase elements to form a poly-phase forcer. U.S. Pat. No. 3,457,482 to Sawyer subsequently disclosed a hybrid variable reluctance design that incorporated permanent magnets. This design used two identical single-phase electromagnets to form a functional forcer. Next Bruce Sawyer disclosed (U.S. Pat. No. 3,836,835) an additional hybrid linear forcer design wherein one permanent magnet and two coils form a functional forcer, the most compact design to date.
Nocito, et al. (U.S. Pat. No. 3,878,411) disclosed an improved linear motor design based on four of the previously disclosed single-phase hybrid variable reluctance elements. This design spaced the hybrid variable reluctance elements in an optimal manner to form a linear motor capable of higher intrinsic positioning accuracy. This design is known as the 2/4-phase motor design since although four single-phase elements are used in each linear motor, the elements typically use coil windings coupled in a particular manner to allow 2-phase drive.
These early patent disclosures typically show large U-shaped permanent magnets coupling the back of two iron cores. This design was common when low-energy product ferrite or alnico magnets were used. With the advent of high-energy product rare earth magnets, a much smaller magnetic volume is required, and it is usually placed between the iron cores. The two designs are sometimes referred to as exterior or back structure vs. interior or sandwich permanent magnet designs. The book Theory and Application of Stem Motors (B. C. Kuo 1974) and Incremental Motion Control Step Motors and Control Systems (B. C. Kuo 1979) and the paper Direct Linear and Cartesian Coordinate Stepping Motors (Ed Pelta, 1985) provide a good technical description of these magnetic designs.
U.S. Pat. No. 6,533,035 to Smith disclosed a design wherein three of the well known single-phase hybrid variable reluctance elements are spaced apart to form a 3-phase linear motor. U.S. Pat. No. 5,742,136 to Ono et al., disclosed a magnetically coupled 3-phase linear motor. Japanese Pat. JP9261944 to Sutoki Riyuutarou disclosed a modified hybrid variable reluctance design wherein one permanent magnet and six coils from a 3-phase linear motor.
To more fully understand the current invention a detailed description of pertinent aspects of prior art is required. It should be noted that most descriptions of Sawyer motor operation approach the subject from an open-loop perspective, assuming the motors are operated as open-loop synchronous machines. As such, the drawings and discussion focus on tooth alignment and natural detent positions. In the discussion that follows the perspective is that of closed-loop control which focuses on force production. Useful force is produced in electromagnet systems when space quadrature of the magnetic fields (M and H vectors) exists. Useful motion occurs when the magnetic fields translate or rotate with time. This leads to the well-known requirements of “space quadrature and time phase” for useful electromechanical energy conversion.